An accelerometer provides an output signal based on an acceleration that acts upon it. Accelerometers are used in a wide variety of applications, such as inertial guidance systems, automobile crash detection systems, video game controllers, and shipping container shock sensors.
An accelerometer with very high sensitivity can be used as a gravity sensor. Gravity sensors are used in such area as geological surveying, oil field exploration, homeland security, and seismology.
The advent of Micro-Electro Mechanical Systems (MEMS) technology has ushered in a new era of accelerometers. A typical MEMS-based accelerometer includes a sensing element that is based on a proof mass attached to a reference frame by means of resilient tether (e.g., a spring element or spring system). An acceleration of the reference frame or the presence of a gravitational field causes a displacement of the proof mass relative to the reference frame. A transducer converts this displacement into an output signal.
The sensitivity of an accelerometer (expressed in terms of Volts or Amperes per unit of acceleration) is dependent upon its signal-to-noise ratio (SNR) and output drift. In the absence of an acceleration, the output signal of the accelerometer typically exhibits a constant voltage or current level. This steady-state output value is referred to as a D.C. bias. Unfortunately, DC bias is subject to drift over time and temperature. In addition, accelerometers are subject to sensitivity drift with temperature. Such drifts contribute to the noise on the output signal, thereby decreasing the signal-to-noise ratio of the accelerometer. This adversely affects the overall sensitivity of the device.
Approaches to mitigating the effects of bias stability have been developed in the prior art. In some cases, temperature sensors and microcontrollers are integrated with the accelerometer. During operation, the temperature-induced drift can be removed from the output signal by means of a look-up table or mathematical solution. Unfortunately, this leads to significant added cost and complexity.
Other approaches rely on the integration of analog correction circuitry with the accelerometer. Unfortunately, it is very hard to tune the correction circuitry to accommodate variations from accelerometer to accelerometer.
Still other approaches employ active temperature stabilization. In such approaches, the accelerometer is held at a temperature above the ambient by means of a heating element. This dramatically increases power consumption for the accelerometer however. Low power consumption is particularly desirable for mobile applications.
In some cases, a user-employable “reset” button is provided to enable a user to zero the accelerometer when it is in a state of zero acceleration. This, however, is impractical in many applications.
Finally, AC-coupling an accelerometer can eliminate bias drift all-together. Unfortunately, in many applications DC-coupling is required or highly desirable (e.g., tilt sensors, geological surveying, inertial navigation, etc.).